The purpose of this grant request is to obtain a high resolution Philips 420 electron microscope with cold stage, eucentric goniometer and low dose kit. Our research programs in structural biology involve computerized image analysis of electron micrographs, coordinated with X-ray crystallography and biochemical techniques. To make full use of these capabilities, it is essential that we upgrade our electron microscope facilities. Our present microscope, a Philips 301, is overloaded with users and its performance is deteriorating due to age-related wear. Technological advances have been made in the field of high resolution electron microscopy and in electron microscope design; in particular, the emergence of cryomicroscopy and the development of improved lenses and goniometer stages. The four major programs requiring a new electron microscope are: (1) Structural Studies of Biological Complexes and Structural Studies of Cellular Actin; (2) Assembly of Viruses, Membranes and Tissue; (3) Muscle Structure and the Contractile Mechanism; and (4) Physical-chemical Basis for the Contractile Mechanism. There are eleven individual research projects involving twenty investigators that depend heavily on the use of an electron microscope. In each of these research projects, the primary goal is to determine the three-dimensional structure of organized macromolecular systems and to understand their functions in terms of dynamic states of their structures. Specimens that have been dehydrated and contrasted with heavy metals suffer various distortions, and are not suitable for high resolution structure analysis. A low-temperature stage would allow samples to be examined in an unstained frozen-hydrated state; an eucentric goniometer stage permits collection of high resolution images from tilted specimens for three-dimensional structure determination. Low electron doses are required to minimize radiation damage. The Philips 420 microscope has the full range of capabilities required for progress in our research programs in structural biology.